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Wed, 4 Apr 2018 11:38:29 -0400 Date: Wed, 4 Apr 2018 17:35:34 +0200 From: Peter Zijlstra To: Kirill Tkhai Cc: mingo@redhat.com, stable@vger.kernel.org, linux-kernel@vger.kernel.org, Waiman Long , Boqun Feng Subject: Re: [PATCH] locking/qrwlock: Give priority to readers with irqs disabled to prevent deadlock Message-ID: <20180404153534.GI4043@hirez.programming.kicks-ass.net> References: <152285543936.4080.5025412328012618076.stgit@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <152285543936.4080.5025412328012618076.stgit@localhost.localdomain> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Wed, Apr 04, 2018 at 06:24:39PM +0300, Kirill Tkhai wrote: > The following situation leads to deadlock: > > [task 1] [task 2] [task 3] > kill_fasync() mm_update_next_owner() copy_process() > spin_lock_irqsave(&fa->fa_lock) read_lock(&tasklist_lock) write_lock_irq(&tasklist_lock) > send_sigio() ... > read_lock(&fown->lock) kill_fasync() ... > read_lock(&tasklist_lock) spin_lock_irqsave(&fa->fa_lock) ... > > Task 1 can't acquire read locked tasklist_lock, since there is > already task 3 expressed its wish to take the lock exclusive. > Task 2 holds the read locked lock, but it can't take the spin lock. > > The patch makes queued_read_lock_slowpath() to give task 1 the same > priority as it was an interrupt handler, and to take the lock That re-introduces starvation scenarios. And the above looks like a proper deadlock that should be sorted by fixing the locking order.